Researches on imaging apparatuses for transmitting light emitted from a light source such as a laser to a subject in the body of the subject and acquiring information of the inside of the subject has been actively done mainly in the medical field. In such imaging techniques, photoacoustic tomography (PAT) has been proposed.
The PAT is a technology for visualizing information relating to optical characteristics of the inside of a living body (i.e., a subject). In the PAT, the living body (subject) is irradiated with pulse light generated from a light source, an acoustic wave generated by the light transmitted and diffused in the living body and absorbed by the body tissue is received, and the received acoustic wave is analyzed and processed. By the PAT, information of the living body such as optical characteristic value distribution in the subject, especially, light energy absorption density distribution can be acquired.
In the PAT, an initial sound pressure P0 of the acoustic wave generated from the light absorber in the subject can be expressed by the following expression:P0=Γ·μa·Φ  expression 1where Γ is a Gruneisen coefficient. The Gruneisen coefficient is obtained by dividing the product of a coefficient of volumetric expansion β and the squared sound speed c by the specific heat at constant pressure Cp. It is known that the coefficient Γ takes a substantially constant value if the subject is specified. μa is a light absorption coefficient, and Φ is a light amount (light amount emitted to an absorber, and also referred to as light influence) in a local region.
Time change of the sound pressure P that is a level of an acoustic wave transmitted inside the subject is measured, and based on the measurement result, initial sound pressure distribution is calculated. The calculated initial sound pressure distribution is divided by the Gruneisen coefficient Γ to obtain distribution of the products of μa and Φ, that is, light energy absorption density distribution.
As shown in the expression 1, in order to obtain the distribution of the light absorption coefficient μa from the distribution of the initial sound pressure P0, it is necessary to obtain the distribution of the light amount Φ in the subject. If it is assumed that when a region large enough to the thickness of the subject is evenly irradiated with light of a certain level, the light transmits inside the subject like a plane wave, the distribution Φ of the light amount in the subject can be expressed as follows:Φ=Φ0·exp(−μeff·d)  expression 2where, μeff is an average equivalent damping coefficient of the subject, and Φ0 is the amount (light amount on a surface of the subject) of the light that enters in the subject from the light source. Moreover, d is a distance from the region (light irradiation region) on the surface of the subject irradiated with the light from the light source to the light absorber in the subject.
According to patent document 1, a living body is evenly irradiated with light under a plurality of conditions, an average equivalent damping coefficient μeff of the subject is calculated, light amount distribution Φ in the subject is calculated by the expression 2, and using the light amount distribution Φ, light absorption coefficient distribution μa in the subject can be obtained by the expression 1.